1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a laser printer, which transfers a toner image born on an image bearing member onto a recording material by using an electrophotographic technique. More specifically, the present invention relates to an image forming apparatus including a transfer belt that performs transfer and conveyance for a recording material.
2. Description of the Related Art
In an electrophotographic apparatus in which a recording material is born and conveyed by a transfer belt stretched by a plurality of rollers, the recording material on the transfer belt is electrostatically attracted onto the transfer belt after passing through a transfer nip portion.
However, if the rigidity of the recording material is low, the recording material cannot be separated from the transfer belt only by utilizing the curvature of a separation roller for stretching the transfer belt and the rigidity of the recording material. That is, the recording material remains stuck on the transfer belt at a position of the separation roller, and this causes separation failure. Accordingly, in Japanese Patent Laid-Open No. 9-015987, as a structure for undulating the transfer belt at the separation position, a method for separating a recording material by protrusions uniformly formed on a surface of a separation roller for stretching a transfer belt is known. While the transfer belt can be undulated at the separation position by using this structure, a great local tension constantly acts on the transfer belt. As a result, transfer ability is made unstable by the influence of resistance variations due to local wear of the transfer belt.
Japanese Patent Laid-Open No. 5-119636 describes a method for deforming a cylindrical transfer-material bearing sheet, which bears a recording material, for the purpose of separating the recording material while reducing wear due to deformation. Japanese Patent Laid-Open No. 5-119636 describes a structure such that rollers are provided as push-up means capable of moving between a position to push up the transfer sheet from the inner side and a position not to push up the transfer sheet. In the method described in Japanese Patent Laid-Open No. 5-119636, the recording material is separated by pushing up the transfer sheet by the rollers, but the transfer sheet is not pushed up while the recording material is not separated. Japanese Patent Laid-Open No. 5-341664 describes a method for separating a thin recording material by a large push-up amount and separating a thick recording material by a small push-up amount in order to separate recording materials of various thicknesses without deforming a transfer-material sheet more than necessary.
When such a structure is applied to a transfer belt, push-up means capable of locally pushing up the transfer belt in a separation step is provided downstream, in a recording-material conveying direction, of a transfer member for transferring a toner image onto a recording material on the transfer belt. When the recording material has low rigidity like thin paper, the transfer belt that is being locally pushed up conveys the recording material so as to undulate the recording material and to thereby increase the stiffness of the recording material during the separation step.
However, while the push-up amount for separating the thick recording material is smaller than the push-amount for separating the thin recording material, it needs to be at least a predetermined push-up amount in order to perform separation by the local push-up motion. A protrusion formed locally in the width direction on the transfer belt by such push-up motion applies, to the transfer belt, a local load that cannot be disregarded. This accelerates local wear of the transfer belt.
Meanwhile, it is found that a thick recording material can be separated from the transfer belt by using the curvature of a stretching member for stretching the transfer belt and high rigidity of the recording material.
Accordingly, to separate a thick recording material, a structure in which the stretching member is located downstream of the transfer member and upstream or downstream of the push-up means in the conveying direction of the recording material is required. When the stretching member is provided upstream of the push-up means in order to separate a thick recording material, a recording-material guide, which guides the recording material separated from the transfer belt to a fixing device located further downstream in the recording-material conveying direction, is provided downstream of the push-up means in the recording-material conveying direction. In this case, a position where the thick recording material separates from the transfer belt becomes farther from a leading end of the recording-material guide, and the thick recording material may deviate from the separation position on the transfer belt because of the high rigidity of the recording material and the conveying force of the belt. When the thick recording material deviates, the degree of deviation varies according to the recording material, and therefore, the conveying direction fluctuates. In contrast, even if a position where a thin recording material separates from the transfer belt becomes farther from the leading end of the recording-material guide, the amount of deviation of the thin recording material does not increase. Hence, the conveying direction will not fluctuate.
Accordingly, the present invention provides an image forming apparatus that performs separation of a thin recording material from a transfer belt and separation of a thick recording material from the transfer belt at different positions and that achieves both stabilization of conveyability of the thin recording material after separation and stabilization of conveyability after separation of the thick recording material separated by a member different from a member for separation of the thin recording material.